Effect of wall roughness on the heat transfer in rotating Rayleigh-Bénard convection*

This study focuses on the heat transfer in rotating Rayleigh‒Bénard convection in the presence of horizontal walls with pyramid-shaped roughness elements. Measurements are performed at a Rayleigh number of 2.2×10⁹ and a Prandtl number of 6.2. The effect of the roughness elements on the heat transfer depends on their size, k, relative to the thickness of the Ekman boundary layer, δ_E. The Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in case of smooth walls, is unaffected by the roughness when δ_E>>k. As the rotation rate increases (and hence δ_E decreases), the roughness elements penetrate the radially inward boundary layer flow into the columnar Ekman vortices, generating more thermal structures and further enhancing the heat transfer. This enhancement continues to increase with decreasing δ_E until δ_E ≈ k. As δ_E decreases further below k, the heat transfer starts decreasing as the strong perturbation of the Ekman boundary layer suppresses the Ekman pumping mechanism. The mechanism is perhaps re-established when δ_E becomes sufficiently smaller than k, i.e., when the surfaces of the roughness elements act locally like sloping walls on which Ekman layers can be formed.